Steerable drilling system

ABSTRACT

A bias unit comprising at least one bias pad  20  moveable, by a piston  36,  between retracted and extended positions, wherein the piston  36  is of non-circular effective cross-sectional shape.

This invention relates to a steerable drilling system for use in the formation of subterranean boreholes, for example for subsequent use in the extraction of hydrocarbons. The invention relates, in particular, to a bias unit for use in such a drilling system.

A known form of steerable drilling system includes a bias unit having a plurality of bias pads pivotally mounted thereon. Each bias pad is moveable between a retracted position and a radially extended position. In the extended position, the bias pad is forced into engagement with the surface of the borehole being formed, thereby applying a sideways acting bias load to the bias unit. The bias load applied to the bias unit is transmitted to a drill bit connected to the bias unit so as to urge the drill bit away from the axis of the part of the borehole in which the drill bit is located or otherwise alter the orientation of the drill bit, and thereby, over time, cause the formation of a curve in the borehole.

By appropriate control of the bias unit to control which of the bias pads are in their extended positions at any given time, steering of the drill bit to cause the borehole to follow a desired path can be achieved.

The bias pads are typically moved to their extended positions by pistons, the pistons being of generally circular cross-sectional shape and of diameter limited by, for example, the design of the remainder of the bias unit. The pistons are driven, in use, by drilling fluid or mud supplied under pressure to the bias unit.

It has been found that, in some conditions and applications, the magnitude of the bias load achievable with such arrangements is less than desired. In some applications, although an acceptable bias load magnitude may be achievable, this may require the supply pressure of the drilling fluid or mud to be higher than is desirable. Even where an acceptable bias load magnitude can be achieved at an acceptable supply pressure, it may be desirable to achieve a reduction in supply pressure to reduce the forces applied to the piston seals and thereby achieve an increase in seal working life.

It is an object of the invention to provide a bias unit in which the disadvantages set out above are reduced, and to provide a steerable drilling system including such a bias unit.

According to the present invention there is provided a bias unit comprising at least one bias pad moveable, by a piston, between retracted and extended positions, wherein the piston is of non-circular cross-sectional shape.

Conveniently, the piston is of cross-sectional shape having a major axis and a minor axis. The bias pad is conveniently pivotally moveable about a pivot axis, and the piston is preferably arranged such that the major axis lies parallel or substantially parallel to the pivot axis. Such an arrangement is advantageous in that it allows an increase in the cross-sectional area of the piston to be achieved without requiring, for example, an increase in the diameter of the bias unit. As a result, the bias load achievable, for a given drilling fluid pressure, is increased.

The piston maybe of obround cross-sectional shape.

The piston is preferably slidable within a bore of curved or part-toroidal form.

The piston may be attached to or form part of the bias pad. However, arrangements are possible in which this is not the case.

For clarity, references herein to the cross-sectional shape of the piston are to the cross-sectional shape of that part or those parts of the piston which form the effective surface area upon which fluid under pressure acts, in use.

The invention also relates to a steerable drilling system incorporating such a bias unit, the drilling system including a drill bit the position and/or orientation of which is adjustable by the bias unit in use.

The invention will further be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of part of a steerable drilling system;

FIGS. 2, 3 and 4 are views of parts of the bias unit of the steerable drilling system of FIG. 1; and

FIG. 5 is a sectional view through the parts shown in FIGS. 2, 3 and 4 when assembled.

The drilling system illustrated, diagrammatically, in FIG. 1 comprises a drill bit 10 arranged to be rotated about its axis by, for example, a fluid powered motor 12. As illustrated, the motor 12 is carried by part of a bias unit 14. Operation of the bias unit 14 is controlled by a control unit 16 which controls the operation of a valve arrangement located within a valve housing 18.

Although only a few components of the downhole steerable drilling system are illustrated in FIG. 1, it will be appreciated that a number of additional components may be provided. For example, stabiliser units may be provided to hold certain parts of the downhole assembly in a chosen position or orientation. Further, a range of sensor devices could be provided, if desired.

The bias unit 14 comprises a housing upon which is pivotally mounted a series of bias pads 20. Each bias pad 20 is pivotally movable between a retracted position and a radially extended position, movement of each pad from the retracted position to the radially extended position being achieved by means of a piston arrangement 22 associated with the respective bias pad 20. Although FIG. 1 only illustrates the provision of two bias pads, it will be appreciated that three or more such pads will typically be provided.

Referring next to FIG. 2 there is shown a clamp plate 24 which forms part of the outer periphery of the housing of the bias unit 14. The clamp plate 24 includes knuckles 26 adapted to receive a pivot pin 28 (see FIG. 5) to allow the pivotal connection of an associated one of the bias pads 20 thereto. The clamp plate 24 further includes a bore 30 which, as shown in FIG. 5, is curved in cross-section so as to be of part-toroidal form, the centre of curvature of the part-toroidal bore 30 being the pivot axis 32.

As shown in FIG. 3, the bias pad 20 is also shaped to include knuckles 34 adapted to receive parts of the pivot pin 28 to establish the pivotal connection between the pad 20 and the clamp plate 24. The pad 20 further includes a projection defining a piston 36 which, in use, is received within the bore 30 to form the piston arrangement 22. The piston 36 carries a seal member 38 so as to form a seal between the piston 36 and the wall defining the bore 30. The pad 20 and piston 36 further define a flowpath 40 whereby fluid supplied to the toroidal bore 30 can escape therefrom, the flowpath 40 being defined, in part, by an orifice formed in a flow restrictor plate 42 located within the piston 36 and arranged so as to ensure that the rate at which fluid can escape from the toroidal bore 30 is restricted to a predetermined rate.

In use, in order to move a pad 20 to its extended position, drilling fluid under pressure is supplied to the appropriate bore 30 to apply a force to the associated piston 36. Subsequently, the supply of fluid under pressure is broken. Fluid escapes from the bore 30 along the flowpath 40 at a restricted rate, thereby allowing the fluid pressure within the bore 30 to fall and the engagement between the pad 20 and the wall of the borehole urges the pad 20 back toward its retracted position.

The supply of fluid under pressure to the bore 30 is controlled by the control unit 16 and valves located within the valve housing 18. Typically, in use, the bias unit 14 rotates and so, in order to maintain a bias load in a substantially constant direction over a period of time, the bias pads must be moved to their extended positions in turn at a speed related to the speed of rotation of the bias unit.

As illustrated, the piston 36 and the toroidal bore 30 are not of circular cross-section, but rather have a cross-section taking the form of a pair of semi-circles separated by a region of rectangular or square cross-section, and thus is of obround or obround-like cross-sectional shape. This is apparent from FIG. 2 and from the shape of the seal shown in FIG. 4 which conforms to the shape of the toroidal bore 30. The obround cross-sectional shape of the seal 38, piston upon which the seal 38 is mounted, and bore 30 thus defines a major axis 44 and a minor axis 46. The major axis lies parallel to the pivot axis 32. As a result, the overall effective cross-sectional area of the piston can be increased, compared to conventional arrangements without requiring an increase in the width of the clamp plate 24, thereby avoiding requiring an increase in the diameter of the bias unit 14. By increasing the effective cross-sectional area of the piston in this way, an increase in the magnitude of the bias load achievable using the bias unit 14 for a predetermined fluid pressure can be attained. Depending upon the application in which the bias unit 14 is used, the magnitude of the bias load achievable may be increased, or alternatively, the bias unit 14 may be operated at a reduced drilling fluid pressure. A further benefit, where a reduced operating pressure is used, is that the seal 38 may be of increased useful working life. As a result, servicing and maintenance of the bias unit 14 may be required at less frequent intervals.

Although the description hereinbefore is of an arrangement in which the piston is of obround shape, it will be appreciated that the invention is also applicable to other arrangements. For example, the piston could be of elliptical cross-sectional shape. A number of other modifications and alterations are possible within the scope of the invention.

Further, the invention may be used in steerable drilling systems of a range of other types, for example steerable drilling systems in which the bias unit is separated from the drill bit by a significant distance, a stabiliser and universal joint being located between the bias unit and the drill bit, or one of a range of other steering techniques. 

1. A bias unit comprising at least one bias pad moveable, by a piston, between retracted and extended positions, wherein the piston is of non-circular effective cross-sectional shape.
 2. A unit according to claim 1, wherein the piston is of effective cross-sectional shape having a major axis and a minor axis.
 3. A unit according to claim 2, wherein the bias pad is pivotally moveable about a pivot axis, and the piston is arranged such that the major axis lies substantially parallel to the pivot axis.
 4. A unit according to any of claim 1, wherein the piston is of obround effective cross-sectional shape.
 5. A unit according to claim 1, wherein the piston is slidable within a bore of curved or part-toroidal form.
 6. A unit according to claim 1, wherein the piston is attached to or forms part of the bias pad.
 7. A steerable drilling system incorporating a bias unit as claimed in claim 1, the drilling system including a drill bit the position and/or orientation of which is adjustable by the bias unit in use. 